Blood Clotting in Pregnancy and COVID-19

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Also known as coagulation, blood clotting is a very important issue in connection with many medical situations, including pregnancy. A clot, also called a thrombus, can obstruct a blood vessel at the location where the clotting process has initiated. Additionally, whether or not a thrombus obstructs the vessel in which it forms, pieces of the thrombus can dislodge and travel within the bloodstream. A traveling blood clot is one type of embolus, an entity that travels in the bloodstream but shouldn’t be there. If an embolus gets stuck within a blood vessel, it forms an embolism –an obstruction that stops blood flow.

There is a kind of Goldilocks zone in the clotting process. If a person clots too easily, the risk of clots forming and obstructing blood vessels increases, but if the tendency to form clots decreases too much the risk of hemorrhage jumps up. The lowest overall risk to health comes when the coagulation system is tweaked just right –enough clotting but not too much, like the porridge of the baby bear not being too hot or too cold for Goldilocks, or the bed not being too hard or too soft.

Pregnancy changes the settings of the coagulation system and, based on what researchers are learning, so does the SARS-Cov2 virus, the virus that causes COVID-19. During pregnancy and during the post partum period, the clotting system adjusts toward a tendency for increased clotting. This helps compensate for blood loss during delivery, but the speedier clotting can be harmful in some women. Classically, doctors have been most worried about clots forming deep veins. This condition, called deep venous thrombosis, can produce an embolus that travels to the lungs, causing an immediately life threatening condition called a pulmonary embolism.

In the case of COVID-19, often patients with worsening disease show elevated levels in their blood of a special protein called D-dimer, which indicates an increased amount of clotting. Scientists think this may be related to the SARS-CoV2 virus interfering with a protein called the ACE-2 receptor. From previous posts, you may remember that the virus is able to attach to the ACE-2, triggering a process that allows the virus to invade the cell. This can happen in lung cells and in various types of cells throughout the body, but ACE-2 is not just sitting around waiting to double-cross the very cell that has given it a home by opening the gates for a coronavirus. ACE-2 actually has a day job that it’s supposed to be doing to help the body, but the virus attaching prevents it from doing this job. This leads to a cascade of problems, one of which appears to be the disruption of the clotting system.

With both pregnancy and COVID-19, the changes in coagulation are complex. In the case of pregnancy, the fact that blood slows down and sometimes can stagnate in deep veins, due to pressure from the growing womb, is a factor, but there are also hormonal changes affecting a cascade of events that controls clotting. Both pregnancy and COVID-19 also involve changes in a protein called von Willebrand factor (vWF). The job of vWF is to hold onto another protein called Factor VIII until the Factor VIII is needed to help clot blood to stop bleeding, and also to help clotting cells, called platelets, stick together. Some women normally do not make enough vWF, due to a genetic disorder, called von Willebrand disease. Women with von Willebrand disease often bleed too easily. In pregnancy, the production of vWF increases, causing women with von Willebrand disease to get better, but the increased vWD may also contribute to the increased tendency to clot in women who don’t have von Willebrand disease. Initially, vWD is produced as a long stringy protein that is supposed to be chopped into smaller pieces by –you guessed it– yet another protein. When vWD is not chopped up, it acts as a stronger clotting agent than it does in its chopped up form, because it sticks more platelets together at once. This relates to COVID-19 for a couple of reasons. One reason is that recent research suggests that, as a result of preventing ACE-2 from doing its job, the SARS-CoV2 virus causes vWF to be released from cells that line blood vessels and also prevents this vWF from being chopped up. Another reason is that people with blood type O tend to chop up vWF and remove it from the body more quickly than people with other blood types. Meanwhile, research suggests that people with blood type O are also less likely than others to develop serious complications of COVID-19, if they are infected with the SARS-CoV2 virus.

Putting all of this together, a similar phenomenon may be occurring with the blood of pregnant women and with people suffering from COVID-19. Does this mean that pregnancy would put you at increased risk of serious complications from COVID-19? Not necessarily. Right now, it’s early in the game and there are many unknowns, so a lot of research lies ahead. Does it mean that your blood type may be important in terms of understanding the risks if you develop COVID-19? Also, not necessarily, but based on what is known there is a connection between blood type O and clotting, due to the accelerated removal of vWF. Consequently, we will explore blood types and pregnancy in a future post.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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